1. Field of the Invention
This invention relates in general to refrigeration circuits and more particularly to a defrost system for use in a refrigeration circuit such as may be incorporated in air conditioning apparatus including a heat pump.
2. Prior Art
The conventional refrigeration circuit employs a compressor, condenser, expansion means and evaporator connected to form a refrigerant flow circuit. The compressor raises the pressure and temperature of gaseous refrigerant and the gaseous refrigerant is then conducted to the condenser wherein it gives off heat to a cooling fluid and is condensed to a liquid. The liquid refrigerant then flows through an expansion means such that its pressure is reduced and is therefor capable of changing from a liquid to a gas absorbing heat during the change in state. A complete change of state from a liquid to a gas occurs in the evaporator and heat is removed from the media flowing in heat transfer relation with the evaporator. Gaseous refrigerant from the evaporator is then conducted back to the compressor.
Under appropriate ambient conditions, the media flowing in heat transfer relation with the evaporator, typically air, has its temperature lowered below its dew point. Once the temperature of the air is below the dew point, moisture is deposited on the coil surfaces resulting in a collection of fluid thereon. If the ambient temperature conditions are sufficiently low or if the temperature of the evaporator is sufficiently low, then ice is formed on the heat exchanger surfaces. Once this ice or frost coats the surfaces of the heat exchanger, the efficiency of the heat exchanger is impaired and overall system efficiency decreases. Consequently, it is desirable to maintain the evaporator surfaces free from ice or frost.
The formation of ice or frost on the heat exchanger surface is particularly accute with heat pumps used to provide heating to an enclosure. In the operation of the heat pump in the heating mode, the outdoor coil functions as an evaporator such that heat may be absorbed from the outside air. If the outside air is at a low temperature, the evaporator must operate at an even lower temperature and consequently it may operate under the appropriate environmental conditions such that ice and frost are formed thereon.
Many systems have been developed for defrosting heat exchanger coils. These include supplying electric resistance heat to the coil surface to melt the ice and reversing the refrigeration system such that hot gas discharged from the compressor is circulated through the evaporator to melt the ice thereon. The inconvenience accompanying reversing the system is that heat is removed from the enclosure to supply heat energy for defrost.
Nonreverse defrost systems, systems which do not include a reversal in the flow path of refrigerant through the refrigeration circuit, have been previously utilized and are disclosed in the art. Most of these systems concern bypassing the condenser such that hot gas from the compressor is discharged directly into the evaporator and then some method is used to vaporize the refrigerant which has liquified in the evaporator in order to maintain superheat in the refrigerant so that it never changes from a gas to a liquid.
In the present defrost system, a combination of reverse and nonreverse defrost is utilized to provide for effective frost removal from the heat exchanger. A three-way valve is mounted in series with a four-way valve such that the four-way valve is utilized to direct refrigerant flow to operate the system in either the heating or cooling mode of operation. Typically defrost of the outdoor heat exchanger of a heat pump is accomplished by operating the heat pump in the cooling mode such that heat energy is supplied by hot gaseous refrigerant from the condenser directly to the outdoor heat exchanger serving as the evaporator during the heating mode. Consequently, the operation of the refrigeration system is reversed and the indoor heat exchanger which should be supplying heat during the heating mode is acting as an evaporator and removing heat from the enclosure to be conditioned.
Herein a three-way valve is provided between the compressor and the four-way valve such that hot gaseous refrigerant from the compressor is either discharged to the four-way valve or discharged directly to the heat exchanger to be defrosted. An intermediate header conducts the hot gaseous refrigerant via feeder tubes into each circuit of the outdoor heat exchanger. The intermediate header further serves during normal operation to conduct the refrigerant between the circuits of the heat exchanger when it is serving as a condenser and as a part of the refrigerant flow path when the heat exchanger is operating as an evaporator, said intermediate header connecting the expansion means to the circuits of the heat exchanger. The three-way valve is energized to supply heat energy directly from the compressor to the outdoor heat exchanger such that gaseous refrigerant is circulated between the outdoor heat exchanger and the compressor for a predetermined time period. If defrost is not accomplished within that time period then the three-way valve is repositioned such that hot gaseous refrigerant is provided to the reversing valve which is then switched to the cooling mode to complete defrost of the heat exchanger. A solenoid valve is provided in the liquid line between the indoor heat exchanger and the outdoor heat exchanger such that during the initial defrost mode with the three-way valve being repositioned to direct hot gaseous refrigerant directly to the outdoor heat exchanger, refrigerant flow between the indoor heat exchanger and the outdoor heat exchanger is prevented.
The utilization of a two step defrost provides a demand defrost system wherein the first mode of defrost checks to determine if defrost is really necessary as well as melting some ice accumulation. If defrost is not necessary, the temperature of the heat exchanger will rise shortly after the three-way valve is positioned for defrost and defrost will be terminated. Hence, a defrost system is provided which verifys the need for defrost on a periodic basis as well as providing means to accomplish defrost. During this first mode of operation to ascertain the necessity of defrost no heat is removed from the enclosure and heating operations may continue without the reversing valve changing position if no major frost accumulation is detected.